The usual cause has been detected is that once removed a previously created SiteMailbox, a conflict occurs between both mailboxes to the creation of a new one, which causes the URL from the SharePoint Online portal is poorly composed.

To delete incorrect references to the mailbox of SiteMailbox initially created:

1. Open the collection of the site from SharePoint in which the SiteMailbox is not working properly. Delete the app from the SiteMailbox where it is still present.

2. Install SharePoint designer if you do not have it installed. Open the site collection in SharePoint Designer. Under the site object, click on the site at the top of the list and in the ribbon, select the option ‘Site Options’. Delete the following entries in case you are presents in the parameter list: